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class="icon minus" data-v-2976c796><path d="M19,2H5C3.3,2,2,3.3,2,5v14c0,1.7,1.3,3,3,3h14c1.7,0,3-1.3,3-3V5C22,3.3,20.7,2,19,2zM20,19c0,0.6-0.4,1-1,1H5c-0.6,0-1-0.4-1-1V5c0-0.6,0.4-1,1-1h14c0.6,0,1,0.4,1,1V19z"></path><path d="M16,11H8c-0.6,0-1,0.4-1,1s0.4,1,1,1h8c0.6,0,1-0.4,1-1S16.6,11,16,11z"></path></svg><svg version="1.1" xmlns="http://www.w3.org/2000/svg" viewbox="0 0 24 24" class="icon plus" data-v-2976c796><path d="M19,2H5C3.3,2,2,3.3,2,5v14c0,1.7,1.3,3,3,3h14c1.7,0,3-1.3,3-3V5C22,3.3,20.7,2,19,2z M20,19c0,0.6-0.4,1-1,1H5c-0.6,0-1-0.4-1-1V5c0-0.6,0.4-1,1-1h14c0.6,0,1,0.4,1,1V19z"></path><path d="M16,11h-3V8c0-0.6-0.4-1-1-1s-1,0.4-1,1v3H8c-0.6,0-1,0.4-1,1s0.4,1,1,1h3v3c0,0.6,0.4,1,1,1s1-0.4,1-1v-3h3c0.6,0,1-0.4,1-1S16.6,11,16,11z"></path></svg></div></div><div class="items" data-v-2976c796><!--[--><!--[--><a class="VPLink link link" href="/articles/rust/01.%E5%9F%BA%E7%A1%80%E5%85%A5%E9%97%A8.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>01.基础入门</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/02.%E5%9F%BA%E7%A1%80%E8%AF%AD%E6%B3%95.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>02.基础语法</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/03.%E6%8C%87%E9%92%88%E4%B8%8E%E5%86%85%E5%AD%98%E5%88%86%E9%85%8D.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>03.指针与内存分配</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/04.%E7%A8%8B%E5%BA%8F%E7%BB%93%E6%9E%84%E5%92%8C%E5%BC%82%E5%B8%B8%E5%A4%84%E7%90%86.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>04.程序结构和异常处理</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/05.%E6%95%B0%E6%8D%AE%E7%BB%93%E6%9E%84%E4%B8%8E%E6%B3%9B%E5%9E%8B.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>05.数据结构与泛型</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link active" href="/articles/rust/06.trait%E4%B8%8E%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>06.trait与生命周期</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/07.%E7%BC%96%E5%86%99%E6%B5%8B%E8%AF%95.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>07.编写测试</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/08.%E6%9E%84%E5%BB%BA%E5%91%BD%E4%BB%A4%E8%A1%8C%E7%A8%8B%E5%BA%8F.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>08.构建命令行程序</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/09.%E9%97%AD%E5%8C%85%E4%B8%8E%E8%BF%AD%E4%BB%A3%E5%99%A8.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>09.闭包与迭代器</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/10.%E5%8F%91%E5%B8%83%E9%A1%B9%E7%9B%AE.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>10.发布项目</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/11.%E6%99%BA%E8%83%BD%E6%8C%87%E9%92%88.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>11.智能指针</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/12.%E5%B9%B6%E5%8F%91.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>12.并发</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/13.%E9%9D%A2%E5%90%91%E5%AF%B9%E8%B1%A1.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>13.面向对象</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/14.%E4%B8%8D%E5%AE%89%E5%85%A8Rust.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>14.不安全Rust</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/15.%E9%AB%98%E7%BA%A7%E7%89%B9%E6%80%A7.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>15.高级特性</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/16.%E6%89%8B%E5%86%99web%E6%9C%8D%E5%8A%A1%E5%99%A8%E5%92%8C%E7%BA%BF%E7%A8%8B%E6%B1%A0.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>16.手写web服务器和线程池</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rust/17.rust%E5%BC%82%E6%AD%A5%E7%BC%96%E7%A8%8B.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>17.rust异步编程</span><!--]--><!----></a><!----><!--]--><!--]--></div></section></div><div class="group" data-v-f332cb62><section class="VPSidebarGroup collapsible collapsed" data-v-f332cb62 data-v-2976c796><div class="title" role="button" data-v-2976c796><h2 class="title-text" data-v-2976c796>rustweb</h2><div class="action" data-v-2976c796><svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" viewbox="0 0 24 24" class="icon minus" data-v-2976c796><path 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server</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/02.Actix.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>02.Actix</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/03.%E8%BF%9E%E6%8E%A5%E6%95%B0%E6%8D%AE%E5%BA%93.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>03.连接数据库</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/04.%E9%94%99%E8%AF%AF%E5%A4%84%E7%90%86.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>04.错误处理</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/05.%E5%AE%8C%E6%95%B4%E9%A1%B9%E7%9B%AE%E9%87%8D%E6%9E%84.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>05.完整项目重构</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/06.%E6%9C%8D%E5%8A%A1%E5%99%A8%E7%AB%AFweb%E5%BA%94%E7%94%A8.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>06.服务器端web应用</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/07.WebAssembly.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>07.WebAssembly</span><!--]--><!----></a><!----><!--]--><!--[--><a class="VPLink link link" href="/articles/rustweb/08.%E5%8F%91%E5%B8%83.html" style="padding-left:0px;" data-v-2cfe069a data-v-3c355974><!--[--><span class="link-text" data-v-2cfe069a>08.发布</span><!--]--><!----></a><!----><!--]--><!--]--></div></section></div><!--]--></nav></aside><div class="VPContent has-sidebar" id="VPContent" data-v-c6a644e1 data-v-c95df128><div class="VPDoc has-sidebar has-aside" data-v-c95df128 data-v-37ebe389><div class="container" data-v-37ebe389><div class="aside" data-v-37ebe389><div class="aside-curtain" data-v-37ebe389></div><div class="aside-container" data-v-37ebe389><div class="aside-content" data-v-37ebe389><div class="VPDocAside" data-v-37ebe389 data-v-afc4c1a1><!--[--><!--]--><!--[--><!--]--><div class="VPDocAsideOutline" data-v-afc4c1a1 data-v-2865c0b0><div class="content" data-v-2865c0b0><div class="outline-marker" data-v-2865c0b0></div><div class="outline-title" data-v-2865c0b0>On this page</div><nav aria-labelledby="doc-outline-aria-label" data-v-2865c0b0><span class="visually-hidden" id="doc-outline-aria-label" data-v-2865c0b0> Table of Contents for current page </span><ul class="root" data-v-2865c0b0 data-v-1188541a><!--[--><!--]--></ul></nav></div></div><!--[--><!--]--><div class="spacer" data-v-afc4c1a1></div><!--[--><!--]--><!----><!--[--><!--]--><!--[--><!--]--></div></div></div></div><div class="content" data-v-37ebe389><div class="content-container" data-v-37ebe389><!--[--><!--]--><main class="main" data-v-37ebe389><div style="position:relative;" class="vp-doc _articles_rust_06_trait%E4%B8%8E%E7%94%9F%E5%91%BD%E5%91%A8%E6%9C%9F" data-v-37ebe389><div><h1 id="rust入门（六）：trait与生命周期" tabindex="-1">Rust入门（六）：Trait与生命周期 <a class="header-anchor" href="#rust入门（六）：trait与生命周期" aria-hidden="true">#</a></h1><h2 id="trait" tabindex="-1">Trait <a class="header-anchor" href="#trait" aria-hidden="true">#</a></h2><p>Trait又被成为共同行为，和其他oop语言的接口定义类似，用抽象的方式定义一个可以共享的行为，其他类型可以用自己的方式实现这个 Trait。</p><p>通过 <code>trait</code> 关键字可以定义一个 trait ，trait内部可以包含多个函数的定义，但是不要求实现，当然你也可以为 trait 中的方法提供一个默认的实现</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">pub</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">trait</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Testa</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fa</span><span style="color:#89DDFF;">(&amp;</span><span style="color:#A6ACCD;">self</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">i32</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#676E95;">//有默认实现</span></span>
<span class="line"><span style="color:#F78C6C;">pub</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">trait</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Testa</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fa</span><span style="color:#89DDFF;">(&amp;</span><span style="color:#A6ACCD;">self</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">i32</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">        </span><span style="color:#F78C6C;">1</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span>
<span class="line"></span></code></pre></div><p>我们在 定义的类型上使用 <code>for</code> 这个关键字就可以实现对应的 trait ，如果这个 trait 具有默认实现，那么新实现的方法将会覆盖默认实现，这类操作被称为重载</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">pub</span><span style="color:#A6ACCD;"> </span><span style="color:#C792EA;">struct</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Test</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#F78C6C;">pub</span><span style="color:#A6ACCD;"> x</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">i32</span><span style="color:#89DDFF;">,</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F78C6C;">impl</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Testa</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">for</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Test</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">fa</span><span style="color:#89DDFF;">(&amp;</span><span style="color:#A6ACCD;">self</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">i32</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">       self</span><span style="color:#89DDFF;">.</span><span style="color:#A6ACCD;">x</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>要注意的是，需要实现的 trait 和需要实现这个 trait 的数据类型，至少有一个要在本地，这样做可以防止其他人破坏你的代码，被称为 “孤儿原则”。</p><h2 id="trait-参数" tabindex="-1">Trait 参数 <a class="header-anchor" href="#trait-参数" aria-hidden="true">#</a></h2><p>在设定参数时，我们可以指定传入一个实现了某些 trait 的类型的参数，比如下面的例子，可以传入一个实现了 Testa 这个trait 的参数</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">pub</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">tb</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">item</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#F78C6C;">impl</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Testa</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#82AAFF;">println!</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;{}&quot;</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> item</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">fa</span><span style="color:#89DDFF;">());</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>同样，我们的泛型也可以规定其实现指定的 trait ，比如下面的例子，需要 T 的泛型是实现了 Testa 这个 trait 的</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">pub</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">tb</span><span style="color:#89DDFF;">&lt;</span><span style="color:#FFCB6B;">T</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Testa</span><span style="color:#89DDFF;">&gt;(</span><span style="color:#A6ACCD;">item</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">T</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#82AAFF;">println!</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;{}&quot;</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> item</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">fa</span><span style="color:#89DDFF;">());</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>如果需要一个参数实现多个 trait 可以使用 + 符号来定义</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">pub</span><span style="color:#A6ACCD;"> </span><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">tb</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">item</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#F78C6C;">impl</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Testa</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">+</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Testb</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span></span>
<span class="line"></span></code></pre></div><p>这种定义可以使用 <code>where</code> 关键字来简化</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">some_function</span><span style="color:#89DDFF;">&lt;</span><span style="color:#FFCB6B;">T</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Display</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">+</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Clone</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">U</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Clone</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">+</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Debug</span><span style="color:#89DDFF;">&gt;(</span><span style="color:#A6ACCD;">t</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">T</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> u</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">U</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">i32</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#676E95;">//这两种写法效果相同</span></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">some_function</span><span style="color:#89DDFF;">&lt;</span><span style="color:#FFCB6B;">T</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">U</span><span style="color:#89DDFF;">&gt;(</span><span style="color:#A6ACCD;">t</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">T</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> u</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">U</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">i32</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#F78C6C;">where</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">T</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Display</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">+</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Clone</span><span style="color:#89DDFF;">,</span></span>
<span class="line"><span style="color:#A6ACCD;">          </span><span style="color:#FFCB6B;">U</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Clone</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">+</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">Debug</span></span>
<span class="line"><span style="color:#89DDFF;">{</span></span>
<span class="line"></span></code></pre></div><h2 id="生命周期" tabindex="-1">生命周期 <a class="header-anchor" href="#生命周期" aria-hidden="true">#</a></h2><p>Rust 中的每一个引用都有其生命周期（lifetime），也就是引用保持有效的作用域。大部分时候生命周期是隐含并可以推断的，但是引用可能有多种结果的时候，引用的生命周期可能是不一定的，这个时候不能直接推断出生命周期，这个时候就需要我们进行生命周期的标注：</p><p>如下的函数，传入两个字符串切片类型，返回一个字符串切片，根据逻辑，返回值要么指向 x，要么指向 y，但是 Rust 并不知道将要返回的引用是指向 <code>x</code> 或 <code>y</code>，所以这里无法推断出生命周期：</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">main</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> string1 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">long string is long</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">        </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> string2 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">xyz</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#A6ACCD;">        </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> result </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">longest</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">string1</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">as_str</span><span style="color:#89DDFF;">(),</span><span style="color:#A6ACCD;"> string2</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">as_str</span><span style="color:#89DDFF;">());</span></span>
<span class="line"><span style="color:#A6ACCD;">        </span><span style="color:#82AAFF;">println!</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">The longest string is </span><span style="color:#89DDFF;">{}&quot;</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> result</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">longest</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> y</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">if</span><span style="color:#A6ACCD;"> x</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">len</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> y</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">len</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">        x</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">}</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">else</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">        y</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>为了修复这个错误，需要增加泛型生命周期参数来定义引用间的关系以便借用检查器可以进行分析。需要使用生命周期标注 <code>&#39;a</code>，可以将参数和返回值的关联起来，如下的例子，生命周期标注定义完毕后，他们都是与生命周期 <code>&#39;a</code> 存在的一样长的字符串 slice。函数会返回一个同样也与生命周期 <code>&#39;a</code> 存在的一样长的字符串 slice。它的实际含义是 <code>longest</code> 函数返回的引用的生命周期与传入该函数的引用的生命周期的较小者一致。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#676E95;">//定义一个生命周期  &amp;&#39;a i32 </span></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">longest</span><span style="color:#89DDFF;">&lt;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#89DDFF;">&gt;(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> y</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">if</span><span style="color:#A6ACCD;"> x</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">len</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> y</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">len</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">        x</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">}</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">else</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">        y</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>当具体的引用被传递给 <code>longest</code> 时，被 <code>&#39;a</code> 所替代的具体生命周期是 <code>x</code> 的作用域与 <code>y</code> 的作用域相重叠的那一部分。换一种说法就是泛型生命周期 <code>&#39;a</code> 的具体生命周期等同于 <code>x</code> 和 <code>y</code> 的生命周期中较小的那一个。因为我们用相同的生命周期参数 <code>&#39;a</code> 标注了返回的引用值，所以返回的引用值就能保证在 <code>x</code> 和 <code>y</code> 中较短的那个生命周期结束之前保持有效。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#676E95;">//这种情况是不能通过的，因为string2的生命周期小，所以我们的返回值的生命周期就是string2的生命周期范围</span></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">main</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> string1 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">long string is long</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> result</span><span style="color:#89DDFF;">;</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">        </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> string2 </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">xyz</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#A6ACCD;">        result </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">longest</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">string1</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">as_str</span><span style="color:#89DDFF;">(),</span><span style="color:#A6ACCD;"> string2</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">as_str</span><span style="color:#89DDFF;">());</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#82AAFF;">println!</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">The longest string is </span><span style="color:#89DDFF;">{}&quot;</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> result</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>指定生命周期参数的正确方式依赖函数实现的具体功能，只有与返回值相关的参数需要生命周期。如将 <code>longest</code> 函数的实现修改为总是返回第一个参数而不是最长的字符串 slice，就不需要为参数 <code>y</code> 指定一个生命周期。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">longest</span><span style="color:#89DDFF;">&lt;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#89DDFF;">&gt;(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> y</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    x</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>而且 Rust 也不允许我们创建一个悬垂引用，如果它指向一个函数内部创建的值，它将会是一个悬垂引用，因为它将会在函数结束时离开作用域。尝试考虑这个并不能编译的 <code>longest</code> 函数实现：</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">longest</span><span style="color:#89DDFF;">&lt;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#89DDFF;">&gt;(</span><span style="color:#A6ACCD;">x</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">,</span><span style="color:#A6ACCD;"> y</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> result </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">really long string</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#A6ACCD;">    result</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">as_str</span><span style="color:#89DDFF;">()</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>如果我们将定义包含引用的结构体，这需要为结构体定义中的每一个引用添加生命周期注解。如果下面例子，这个注解意味着 <code>ImportantExcerpt</code> 的实例不能比其 <code>part</code> 字段中的引用存在的更久。它存放了变量 <code>novel</code> 所拥有的 <code>String</code> 的第一个句子的引用。<code>novel</code> 的数据在 <code>ImportantExcerpt</code> 实例创建之前就存在。另外，直到 <code>ImportantExcerpt</code> 离开作用域之后 <code>novel</code> 都不会离开作用域，所以 <code>ImportantExcerpt</code> 实例中的引用是有效的。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#C792EA;">struct</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">ImportantExcerpt</span><span style="color:#89DDFF;">&lt;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    part</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">,</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">main</span><span style="color:#89DDFF;">()</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> novel </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">String</span><span style="color:#89DDFF;">::</span><span style="color:#82AAFF;">from</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">Call me Ishmael. Some years ago...</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> first_sentence </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> novel</span><span style="color:#89DDFF;">.</span><span style="color:#82AAFF;">split</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&#39;</span><span style="color:#C3E88D;">.</span><span style="color:#89DDFF;">&#39;</span><span style="color:#89DDFF;">).</span><span style="color:#82AAFF;">next</span><span style="color:#89DDFF;">().</span><span style="color:#82AAFF;">expect</span><span style="color:#89DDFF;">(</span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">Could not find a &#39;.&#39;</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">);</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> i </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">ImportantExcerpt</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">        part</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> first_sentence</span><span style="color:#89DDFF;">,</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">};</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><p>当为带有生命周期的结构体实现方法时。结构体字段的生命周期必须总是在 <code>impl</code> 关键字之后声明并在结构体名称之后被使用，因为这些生命周期是结构体类型的一部分。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">impl</span><span style="color:#89DDFF;">&lt;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">ImportantExcerpt</span><span style="color:#89DDFF;">&lt;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#89DDFF;">&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">level</span><span style="color:#89DDFF;">(&amp;</span><span style="color:#A6ACCD;">self</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">i32</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"><span style="color:#A6ACCD;">        </span><span style="color:#F78C6C;">3</span></span>
<span class="line"><span style="color:#A6ACCD;">    </span><span style="color:#89DDFF;">}</span></span>
<span class="line"><span style="color:#89DDFF;">}</span></span>
<span class="line"></span></code></pre></div><h2 id="生命周期省略" tabindex="-1">生命周期省略 <a class="header-anchor" href="#生命周期省略" aria-hidden="true">#</a></h2><p>Rust 团队发现在特定情况下 Rust 程序员们总是重复地编写一模一样的生命周期注解，这些场景是可预测的并且遵循几个明确的模式，被编码进 Rust 引用分析的模式被称为 <strong>生命周期省略规则</strong>（<em>lifetime elision rules</em>）。规则如下：</p><ol><li>有一个引用参数的函数有一个生命周期参数：<code>fn foo&lt;&#39;a&gt;(x: &amp;&#39;a i32)</code>，有两个引用参数的函数有两个不同的生命周期参数，<code>fn foo&lt;&#39;a, &#39;b&gt;(x: &amp;&#39;a i32, y: &amp;&#39;b i32)</code>，依此类推。</li><li>只有一个输入生命周期参数，那么它被赋予所有输出生命周期参数：<code>fn foo&lt;&#39;a&gt;(x: &amp;&#39;a i32) -&gt; &amp;&#39;a i32</code>。</li><li>第三条规则是如果方法有多个输入生命周期参数并且其中一个参数是 <code>&amp;self</code> 或 <code>&amp;mut self</code>，说明是个对象的方法(method)，那么所有输出生命周期参数被赋予 <code>self</code> 的生命周期。</li></ol><p>如下面的例子：开始时签名中的引用并没有关联任何生命周期：</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">first_word</span><span style="color:#89DDFF;">(</span><span style="color:#A6ACCD;">s</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"></span></code></pre></div><p>接着编译器应用第一条规则，也就是每个引用参数都有其自己的生命周期。</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">first_word</span><span style="color:#89DDFF;">&lt;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#89DDFF;">&gt;(</span><span style="color:#A6ACCD;">s</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;</span><span style="color:#FFCB6B;">str</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"></span></code></pre></div><p>对于第二条规则，因为这里正好只有一个输入生命周期参数所以是适用的。第二条规则表明输入参数的生命周期将被赋予输出生命周期参数，所以现在签名看起来像这样：</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#F78C6C;">fn</span><span style="color:#A6ACCD;"> </span><span style="color:#82AAFF;">first_word</span><span style="color:#89DDFF;">&lt;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#89DDFF;">&gt;(</span><span style="color:#A6ACCD;">s</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#89DDFF;">)</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">-&gt;</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">a</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">{</span></span>
<span class="line"></span></code></pre></div><p>现在这个函数签名中的所有引用都有了生命周期，如此编译器可以继续它的分析而无须程序员标记这个函数签名中的生命周期。</p><h2 id="静态生命周期" tabindex="-1">静态生命周期 <a class="header-anchor" href="#静态生命周期" aria-hidden="true">#</a></h2><p>这里有一种特殊的生命周期：<code>&#39;static</code>，其生命周期<strong>能够</strong>存活于整个程序期间。所有的字符串字面值都拥有 <code>&#39;static</code> 生命周期，我们也可以选择像下面这样标注出来，这个字符串的文本被直接储存在程序的二进制文件中而这个文件总是可用的。因此所有的字符串字面值都是 <code>&#39;static</code> 的：</p><div class="language-rust"><button class="copy"></button><span class="lang">rust</span><pre><code><span class="line"><span style="color:#C792EA;">let</span><span style="color:#A6ACCD;"> s</span><span style="color:#89DDFF;">:</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&amp;&#39;</span><span style="color:#FFCB6B;">static</span><span style="color:#A6ACCD;"> </span><span style="color:#FFCB6B;">str</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">=</span><span style="color:#A6ACCD;"> </span><span style="color:#89DDFF;">&quot;</span><span style="color:#C3E88D;">I have a static lifetime.</span><span style="color:#89DDFF;">&quot;</span><span style="color:#89DDFF;">;</span></span>
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